Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 11.9	$ 0	$ 0	$ 0
Financial Assets
Derivative instruments - short-term
Derivative instruments - long-term
Financial liabilities
Derivative instruments - short-term
Derivative instruments - long-term	0	3.6
Level 3 Unobservable input reconciliation [Roll Forward]
Beginning Balance		0	0
Ending Balance		0	0
Impairment of Oil and Gas Properties	33.5	133.0	218.2
Proved properties, estimated fair values	0	71.9
Proved properties [Member]
Level 3 Unobservable input reconciliation [Roll Forward]
Impairment of Oil and Gas Properties	1.2	107.6	195.5
Fair Value, Measurements, Recurring [Member]
Financial Assets
Derivative Assets	1.2	192.8
Financial liabilities
Derivative Liabilities	26.7	6.2
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0.2	188.7
Derivative instruments - long-term	0.4	4.1
Financial liabilities
Derivative instruments - short-term	24.1	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	0.6
Financial liabilities
Derivative instruments - short-term	2.6	2.6
Derivative instruments - long-term		3.6
Fair Value, Measurements, Recurring [Member] | Carrying Amount [Member]
Financial liabilities
Checks outstanding in excess of cash balances		39.7
Long-term debt	2,997.5	3,206.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial Assets
Derivative Assets	0	0
Financial liabilities
Derivative Liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0	0
Derivative instruments - long-term	0	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	11.9	0
Financial liabilities
Checks outstanding in excess of cash balances	90.9	39.7
Long-term debt	3,034.9	3,420.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Derivative Assets	6.5	193.9
Financial liabilities
Derivative Liabilities	32.0	7.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	5.5	189.7
Derivative instruments - long-term	0.4	4.2
Financial liabilities
Derivative instruments - short-term	29.4	1.0
Derivative instruments - long-term		0.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	0.6
Financial liabilities
Derivative instruments - short-term	2.6	2.6
Derivative instruments - long-term		3.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial Assets
Derivative Assets	0	0
Financial liabilities
Derivative Liabilities	0	0
Level 3 Unobservable input reconciliation [Roll Forward]
Realized gains and losses		0.6
Change in Unrealized Gain (Loss)		3.8
Settlements		(0.6)
Transfers out of Level 3		(3.8)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0	0
Derivative instruments - long-term	0	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Netting Adjustments [Member]
Financial liabilities
Derivative instruments - long-term		(0.1)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Assets	(5.3)	(1.1)
Financial liabilities
Derivative Liabilities	(5.3)	(1.1)
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	(5.3)	(1.0)
Derivative instruments - long-term	0	(0.1)
Financial liabilities
Derivative instruments - short-term	(5.3)	(1.0)
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member] | Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term		$ 0